ofdm_equalizer_simpledfe.h

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/* -*- c++ -*- */
/* Copyright 2012 Free Software Foundation, Inc.
 * 
 * This file is part of GNU Radio
 * 
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 * 
 * GNU Radio is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#ifndef INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H
#define INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H

#include <gnuradio/digital/api.h>
#include <gnuradio/digital/constellation.h>
#include <gnuradio/digital/ofdm_equalizer_base.h>

namespace gr {
  namespace digital {

    /* \brief Simple decision feedback equalizer for OFDM.
     * \ingroup ofdm_blk
     * \ingroup equalizers_blk
     *
     * \details
     * Equalizes an OFDM signal symbol by symbol using knowledge of the
     * complex modulations symbols.
     * For every symbol, the following steps are performed:
     * - On every sub-carrier, decode the modulation symbol
     * - Use the difference between the decoded symbol and the received symbol
     *   to update the channel state on this carrier
     * - Whenever a pilot symbol is found, it uses the known pilot symbol to
     *   update the channel state.
     *
     * This equalizer makes a lot of assumptions:
     * - The initial channel state is good enough to decode the first
     *   symbol without error (unless the first symbol only consists of pilot
     *   tones)
     * - The channel changes only very slowly, such that the channel state
     *   from one symbol is enough to decode the next
     * - SNR low enough that equalization will always suffice to correctly
     *   decode a symbol
     * If these assumptions are not met, the most common error is that the
     * channel state is estimated incorrectly during equalization; after that,
     * all subsequent symbols will be completely wrong.
     *
     * Note that the equalized symbols are *exact points* on the constellation.
     * This means soft information of the modulation symbols is lost after the
     * equalization, which is suboptimal for channel codes that use soft decision.
     *
     */
    class DIGITAL_API ofdm_equalizer_simpledfe : public ofdm_equalizer_1d_pilots
    {
     public:
      typedef boost::shared_ptr<ofdm_equalizer_simpledfe> sptr;

      ofdm_equalizer_simpledfe(
          int fft_len,
          const gr::digital::constellation_sptr &constellation,
          const std::vector<std::vector<int> > &occupied_carriers = std::vector<std::vector<int> >(),
          const std::vector<std::vector<int> > &pilot_carriers = std::vector<std::vector<int> >(),
          const std::vector<std::vector<gr_complex> > &pilot_symbols = std::vector<std::vector<gr_complex> >(),
          int symbols_skipped = 0,
          float alpha = 0.1,
          bool input_is_shifted = true);

      ~ofdm_equalizer_simpledfe();

      void equalize(gr_complex *frame,
                      int n_sym,
                      const std::vector<gr_complex> &initial_taps = std::vector<gr_complex>(),
                      const std::vector<tag_t> &tags = std::vector<tag_t>());

      /*
       * \param fft_len FFT length
       * \param constellation The constellation object describing the modulation used
       *                      on the subcarriers (e.g. QPSK). This is used to decode
       *                      the individual symbols.
       * \param occupied_carriers List of occupied carriers, see ofdm_carrier_allocator
       *                          for a description.
       * \param pilot_carriers Position of pilot symbols, see ofdm_carrier_allocator
       *                          for a description.
       * \param pilot_symbols Value of pilot symbols, see ofdm_carrier_allocator
       *                          for a description.
       * \param alpha Averaging coefficient (in a nutshell, if \f$H_{i,k}\f$ is the channel
       *              state for carrier i and symbol k,
       *              \f$H_{i,k+1} =  \alpha H_{i,k} + (1 - \alpha) H_{i,k+1}\f$. Make this
       *              larger if there's more noise, but keep in mind that larger values
       *              of alpha mean slower response to channel changes).
       * \param symbols_skipped Starting position within occupied_carriers and pilot_carriers.
       *                        If the first symbol of the frame was removed (e.g. to decode the
       *                        header), set this make sure the pilot symbols are correctly
       *                        identified.
       * \param input_is_shifted Set this to false if the input signal is not shifted, i.e.
       *                         the first input items is on the DC carrier.
       *                         Note that a lot of the OFDM receiver blocks operate on shifted
       *                         signals!
       */
      static sptr make(
          int fft_len,
          const gr::digital::constellation_sptr &constellation,
          const std::vector<std::vector<int> > &occupied_carriers = std::vector<std::vector<int> >(),
          const std::vector<std::vector<int> > &pilot_carriers = std::vector<std::vector<int> >(),
          const std::vector<std::vector<gr_complex> > &pilot_symbols = std::vector<std::vector<gr_complex> >(),
          int symbols_skipped=0,
          float alpha=0.1,
          bool input_is_shifted=true
      );

     private:
      gr::digital::constellation_sptr d_constellation;
      //! Averaging coefficient
      float d_alpha;
    };

  } /* namespace digital */
} /* namespace gr */

#endif /* INCLUDED_DIGITAL_OFDM_EQUALIZER_SIMPLEDFE_H */